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Phase change absorbents have recently drawn increasing attention due to the potential of for low energy-consuming CO2 capture. When forming two phases after CO2 absorption, only the CO2-rich phase is heated in the desorption process thus reducing the sensible heat. In this work, five amine and three alcohol blends were used to investigate their phase change behavior after reacting with CO2. Phase separation could easily occur for DETA/alcohol/water combinations after CO2 absorption, which was confirmed by the large difference of the solvation free energy in alcohol and in water (i.e. DETA-CO2 has 12.5 kcal/mol ΔGsol in 1-propanol less than that in water). The optimization of the DETA/1-propanol/H2O solution was confirmed to be 30% DETA + 30% 1-propanol + 40% H2O (D30P30H40). It was found that above 90% of CO2 loading was enriched in lower phase. 13C NMR spectrum indicated that carbamate and bicarbonate was formed in lower phase and 1-propanol acted as a good physical solvent to trigger phase separation and facilitate the reaction between amine and CO2.

Carbon capture and utilization technologies are urged more than ever due to the excessive emissions of CO2 that could lead to serious environmental problems. Phase change absorbents are quite competitive in reducing the energy consumption of carbon capture. In this work, tetramethylammonium glycinate ([N1111][Gly]), an efficient absorbent, was dissolved in ethanol and 1-propanol, respectively, to absorb CO2. After CO2 absorption, the saturated solution separated into two phases. CO2 was enriched in the lower phase, so that only the CO2-rich phase needed to be regenerated and the energy consumption of regeneration could be thus reduced. Under the optimal conditions, the CO2 loading of fresh absorbents could reach 0.85 mol CO2/mol IL. On the basis of the 13C NMR analysis and quantum chemical calculation, a three-step reaction mechanism of CO2 capture with [N1111][Gly]/ethanol was proposed. CO2 reacted with [N1111][Gly] to form carbamate, which was similar to the two-step Zwitterionic mechanism of MEA with CO2. Then the formed carbamate could react with ethanol to form ethyl carbonate and [N1111][Gly], which resulted in higher CO2 loading. Moreover, hydrogen bond characteristics were analysed to clarify the phase change mechanism. The number and bonding energy of intramolecular hydrogen bonds were increased after capturing CO2, leading to the formation of precipitation.